One of the problems encountered in the operational implementation of a near-nondirectional aerial is the modification of its radiation diagram due to reflection effects (multiple paths).
Antennas with the purest possible circular polarization are customarily used to solve this type of problem.
An objective of the invention is to propose a device which allows at the same time pure circularly polarized transmission and omnidirectional coverage.
Numerous types of omnidirectional antennas are already known.
Mention may be made in particular of slot antennas arranged on cylinders, and more particularly two-cone antennas, as well as conical spiral antennas, or alternatively antennas of dipole type, for example those which have been described in the publications:
Brown and Woodward, "Circularly polarized omnidirectional antenna", R.C.A. Rev. June 1947; PA1 K. Sakaguchi and N. Hasebe "Acriculary polarized omnidirectional antenna", IEEE Trans. on Antennas and propagation. PA1 Harold A. Wheeler "A helical Antenna for circular polarisation", Proceedings of I.R.E., December 1947.
These various types of antennas allow toric radiation such as illustrated in FIG. 1, but do not allow the production of satisfactory circular polarization.
For its part, the invention proposes a novel type of transmission device with toric-radiation antenna exhibiting better polarization performance than the toric-radiation antennas of the prior art.
Helical antennas are known for their circularly polarized transmission properties.
In this regard, reference may advantageously be made to the publication:
An omnidirectional antenna with four helical aerial wires has already been proposed in U.S. Pat. No. 5,450,093. These various aerial wires are fed therein out of phase with one another.
However, the radiation diagrams of this type of antenna are still not fully satisfactory.
For its part, the invention proposes a transmission device whose antenna is of the type with helical aerial wires and which exhibits an improved transmission diagram as compared with that of an antenna of the type described in U.S. Pat. No. 5,450,093.